Power MOSFETs can be damaged by overheating, which can be caused by excessive current flows, high ambient temperatures, or a combination of these factors. Monitoring the current flow through the MOSFET is not a satisfactory way of detecting an overtemperature condition because it is difficult to distinguish a short circuit condition, which might create excessive temperatures, from the large transient currents which may occur during the normal switching of the MOSFET. For example, current spikes resulting from the start-up of a motor or incandescent lamp or the charging of a capacitor can falsely trip an overcurrent detect circuit. If the MOSFET is momentarily shut off as a result of a false overcurrent indication, noisy or sporadic turn-off waveforms can result. In some cases, the start-up cycle can repeat indefinitely. When the MOSFET controls a motor, such operation results in a "stuck rotor" condition wherein the motor never reaches adequate torque to overcome stiction. On the other hand, in the event of a "soft short", the current may be large enough to cause overheating but not large enough to trigger the overcurrent detection circuitry.
Assuming a constant supply current, the voltage drop across a forward-biased diode is inversely proportional to the ambient temperature. Hence, it is known to measure temperature by detecting the voltage across a forward-biased diode.
In lateral power MOSFETs a temperature sensing component may be integrated into the substrate, which is often set at some fixed potential such as ground. With vertical power MOSFETs, however, the substrate is normally the drain, whose potential moves up and down. Thus, a temperature detection diode in the drain must move up and down or it will interfere with the operation of the device. Fabricating such a structure and providing the proper drive circuit to sense the voltage even during switching is difficult. Prior art efforts focused on finding ways around this complication.
In some cases, the temperature detection diode is mounted on the same heat sink or metal trace as a vertical power MOSFET. For example, FIG. 1 shows a vertical MOSFET 10 which is mounted together with a temperature detection diode 11 on a leadframe or metal trace 12 which links the drain of MOSFET 10 to the anode of diode 11. A wire 15 connects to the source of MOSFET 10, and a wire 17 connects to the cathode of diode 11. The problem with this arrangement is that, while diode 11 measures the temperature of trace 12, the heat is mainly generated within a thin epitaxial (epi) layer at the top of MOSFET 10. Because the heat generated in the epi layer must flow downward through the substrate, the temperature in the active region of the MOSFET may be substantially higher than the temperature of the trace. As a result, the temperature within the MOSFET 10 may reach dangerous levels without being detected by diode 11. The MOSFET may incur permanent damage before it is shut off.
In FIG. 2, which is a cross-sectional view of a MOSFET 13, the temperature detector is integrated into the MOSFET. MOSFET 13 is a vertical device which includes cells 14A, 14B and 14C formed in an N-epitaxial (epi) region 16. N+ substrate 18 represents the drain of MOSFET 13. Temperature detection diode 20 includes a P region 22 and N region 24 which are typically formed in polysilicon. Diode 20 is electrically isolated from the substrate by a field oxide region 26. The problem with this type of arrangement is that the fabrication of the PN diode in polysilicon adds process steps, complexity, cost and possibly defect-related yield problems. Moreover, a significant thermal gradient (e.g., 30-40.degree. C.) may occur across the field oxide region 26, and therefore this arrangement has some of the same delay problems as the arrangement shown in FIG. 1. Reducing the thickness of the oxide layer cannot entirely eliminate the temperature gradient. Diode 20 may also be sensitive to electrostatic discharges between it and the substrate and to voltage transients within MOSFET 13. Moreover, a region of high electric fields may by created in the underlying silicon, and this can lead to a reduction in the voltage-blocking capability of the device. The polysilicon diode approach has therefore not been successful in the thermal protection of power MOSFETs.
What is needed is a vertical power MOSFET with a reliable integral temperature sensor which does not degrade the device's electrical performance and which can be used in tandem with a control circuit to shut the MOSFET down in the event of an overtemperature condition. In addition, fabricating the temperature sensor should be uncomplicated and should require no or minimal additional masking steps.